A three-dimensional net-like structure formed by bending molten filaments of a synthetic resin material extruded from multi-hole nozzle in random coil shape to be welded to one another and solidified is conventionally used as a core material for cushion.
The cushion, for example, mattress is required to satisfy various needs, such as the recycling efficiency and the low cost, in addition to various conditions for proving a comfortable sleep, for example, the elasticity, the adequate repulsive force, the good maintenance property, vibration damping property, quietness, air permeability and light. It is difficult, however, that the single three-dimensional net-like structure satisfies all these physical properties. A mattress has been proposed to make one surface soft and the other surface hard by stacking two three-dimensional net-like assemblies having different bending densities (Patent Literature 1 and Patent Literature 2). Patent Literature 1, however, does not take into account the lightness and the air permeability and does not teach concrete examples of the material. Patent Literature 2, on the other hand, changes the pitch of holes arrayed in a longitudinal direction of a nozzle in each divisional area, so as to change the density of ejected filaments and integrally mold a multi-density net-like structure. The material used is, however, only polyester elastomer.
A multi-layered net-like structure has been disclosed, in which a thermoplastic elastic resin layer and a thermoplastic non-elastic resin layer are stacked and fused to provide a stacked net-like body having substantially flattened surface, and a non-woven fabric made of continuous fiber is joined with a single surface of the stacked net-like body for the purpose of supporting and reinforcing (Patent Literature 3). This multi-layered net-like structure requires joining the non-woven fabric as the reinforcing layer for supporting the cushion material. This disclosure recommends using polyester for both the thermoplastic elastic resin layer and the thermoplastic non-elastic resin layer that are fused to or joined with each other in terms of recovery and recycling after wasting and avoids the use of polyolefin.
A layered cushion structure has been disclosed, in which a cushion material made of a fiber assembly is placed on a surface layer and a woven fabric providing small cavities for releasing heat and moisture is stacked as a back layer. The layered cushion structure has at least two layers of the cushion material made of the fiber assembly, and the hardness of the respective layers gradually decreases toward the surface layer (Patent Literature 4). This disclosure uses polyether ester elastomer for the cushion material and avoids the use of polyolefin.
A bed mattress has been proposed, in which a wadding layer (hard cotton layer) formed from double raschel knit is stacked on an upper surface of a cushion layer, and the entire surfaces are covered with a case (Patent Literature 5). Stacking the wadding layer provides the function of increasing the degree of freedom following the deformation pressure and moving the contact point supporting the compressive force even in response to a slight change. The cushion layer is a single layer and serves to receive a local external force transmitted from the wadding layer relieve the concentration of stress (enhance the body pressure dispersion property). This configuration is unlikely to provide the sufficient body-holding function and eliminate the uncomfortable sinking feeling. The disclosure suggests using polyester as the material for both the cushion layer and the wadding layer in terms of the recycling efficiency, the heat resistance and the durability. This means that the core material is substantially made of 100% polyester.
The techniques of Patent Literature 2 to Patent Literature 5 have the problem of a significant increase in total weight in order to ensure the repulsive force equivalent to polyolefin, reduce the uncomfortable sinking feeling due to excessive sink and provide the body-holding function.
Various techniques have been proposed for a three-dimensional net-like structure formed by conventional extrusion molding to have filaments partly tangled in loops and a two-surface or four-surface molding method of the three-dimensional net-like structure. One example is described in Patent Literature 6 in order to mold a three-dimensional net-like structure. A proposed method of molding a three-dimensional net-like structure presses down molten filaments made of or mainly made of a thermoplastic synthetic resin from a die equipped with a nozzle having a plurality of holes at an edge, makes the molten filament free fall between endless conveyors that are fully or partly submerged in water, and hauls off the filaments at a lower speed than the falling speed, so as to manufacture a three-dimensional net-like structure. The endless conveyors are arranged to face each other to form a specified shape (for example, quadrilateral) in a direction perpendicular to the extrusion direction. The interval between the endless conveyors facing each other is set to be narrower than the width of an assembly of extruded filaments. Two surfaces or four surfaces of the outer periphery of the filament assembly are brought in contact with the endless conveyors downstream and upstream of the location where the endless conveyors are submerged in water.
The prior art three-dimensional net-like structure has loops formed at random and is shrunk by application of heat. For example, in an application for a mattress, the cover is likely to be wrinkled by repeated high-temperature sterilization. This makes the user feel uncomfortable and may cause bedsore by long-term use. Especially shrinkage of the three-dimensional net-like structure in the longitudinal direction has significant effect and high likelihood of wrinkles.
Additionally, the prior art three-dimensional net-like structure is unlikely to exert the functions according to the characteristics of the human body. For example, in an application of a mattress, the human body has the natural adjustment function of performing posture control during sleep, stretching and changing the body position, relaxing the muscles and the body to equalize the load of the body and reset the body stress. The proposed three-dimensional net-like structure, however, has insufficient structural characteristics following the natural adjustment function. There are a diversity of needs for products using the three-dimensional net-like structure and needs for high quality of products. This causes a difficulty in satisfying both the diversity of needs to be fit for the characteristics of the human body and the needs for the high quality of products.
A cushion material made of urethane foam has excellent durability and body pressure dispersion property of a certain level and is conventionally employed for mattress. This cushion material, however, has a problem of low performance of absorbing shear force, which is noted as one cause of bedsore. Low-resilient urethane foam has been proposed as the material having excellent performance of absorbing shear force. The low-resilient urethane foam, however, has low air permeability, in addition to difficulty in roll-over and a temperature-dependent change in hardness.
Using urethane having the open cell foam for a mattress has been proposed (Patent Literature 7). Patent Literature 7 describes that impregnating the urethane having the open cell foam with a resin having pressure-sensitive adhesiveness provides the excellent performance for trapping and collecting ticks. This urethane, however, originally has low repulsive force, so that even increasing the thickness does not provide the sufficient functions as a mattress.
A layer-structured mattress formed from urethane foam has been proposed, in which a lower layer is made of high-density urethane foam, a middle layer is made of low-resilient urethane foam and an upper layer is made of filter foam having the open cell structure (Patent Literature 8 and Patent Literature 9). This mattress takes into account the repulsive property and the body-holding function but uses the urethane foam of low air permeability for the lower layer and the middle layer. This is likely to cause sweating during sleep and is undesirable in terms of sanitation. A proposed technique to solve this problem has air holes provided in a low-resilient mattress (Patent Literature 10). This technique is, however, insufficient for release of heat and vapor through the side face of the mattress and intake of the outside air.
A three-dimensional net-like structure formed by bending a plurality of molten filaments in random coil shape to be thermally fused to one another has been proposed as an alternative of the urethane foam having low water permeability and likeliness of sweating (Patent Literature 5). Patent Literature 5 has proposed a double raschel knit joined with or stacked on a single surface or respective surfaces of the three-dimensional net-like structure.